1. Priority
Priority is claimed to German Patent Application No. 10 2007 033 300.7, filed Jul. 18, 2007. The aforementioned priority document is incorporated herein by reference in its entirety.
2. Field of the Invention
The field of the present invention relates to multilayer, surface structured solid plates for light guidance. Such light guides are generally formed from a transparent plastics material and optionally one or more cover layers.
3. Background
In the use of diffuser plates in the so-called backlight units (BLUs) of flat panel displays, an important factor is a very high and homogeneous brightness of the overall system, so that the brightness of the image on the flat screen is as high as possible. In principle a backlight unit (direct light system) has the structure described hereinafter. It consists generally of a housing in which, depending on the size of the backlight unit, a variable number of high-voltage tubular lamps, so-called CCFL (cold cathode fluorescent lamps) are generally arranged parallel to one another. The inside of the housing is equipped with a white diffusely light-reflecting surface. In individual cases other light sources can also be used (LEDs), which however does not further influence the basic structure. The diffuser plate, which has a thickness of 1 to 3 mm, preferably a thickness of 1.5 mm to 2.0 mm, lies on this lighting system. A set of films which can have the following functions is located on the diffuser plate: redistribution and focussing of the light in the forward direction, and homogenisation (diffuser films), redistribution and focussing of the light in the forward direction by prismatically structured films, for example by the so-called BEF (brightness enhancing films), in addition to a prism film, often a further diffuser film, for avoiding Moire effects or a reflecting polarizer, for example DBEF (dual brightness enhancement films) and linear polarisers. The linearly polarising film lies directly underneath the LCD display arranged on top.
Light-scattering translucent products of polycarbonate with various light-scattering additives, and moulded parts produced therefrom, are already known from the prior art.
Thus, for example, EP-A 634 445 discloses light-scattering compositions which contain polymeric particles based on vinyl-acrylate with a core/shell morphology in combination with TiO2.
The use of light-scattering polycarbonate films in flat screens is described in US 2004/0066645. Here, polyacrylates, PMMA, polytetrafluoroethylenes, polyalkyl trialkoxysiloxanes and mixtures of these components are mentioned as light-scattering pigments.
In DE 10 2005 039 413 PC diffuser plates containing 0.01% to 20% of scattering pigment are described.
However, all these diffuser plates have a transmission of 50%-70%, whereas polycarbonate not containing additives has a light transmission of 90%. The transmission has a decisive influence on the achievable brightness.
In order to evaluate the suitability of the light-scattering plates for so-called backlight units for LCD flat screens, in particular the brightness of the overall system must be taken into account, in other words the whole BLU including the set of films, and not only the diffuser plates per se. The diffuser plates should have as high a homogeneity as possible of the light distribution at maximum brightness. Both properties are desired, but in the case of conventional diffuser plates they are as a rule conflicting properties.
A homogenisation by means of surface structures as described in JP 2006/284697 or US 2006/10262666 has the advantage of a high transmission and thus a high brightness. In this connection simple barrel-shaped or prism-shaped ridges or a combination thereof form the basic type of structure, which in certain cases contain slight variations such as indentations; mathematically these structures can often be described in terms of elliptical sections and in this case are as a rule termed lenticular structures. However, the achievable homogeneity is limited and is less than the achievable homogeneity of conventional diffuser plates. The structure enlarges the direct image of the lamps. Without going into the theory, the light density maxima are enlarged via the lamps, though their position remains unaltered. A minimum exists between the lamps. For large lamp interspacings and where there are only a few lamps, this leads to an inhomogeneous image. In order to achieve a sufficient homogeneity for demanding BLU structures (larger lamp interspacings, small overall height), complicated and costly multilayer structures are often proposed.
Light-guiding structures in plastics material containing scattering additive consisting of truncated prisms are described in CN 1924620. These structures are said to form three clear images of the lamps and are enlarged by means of the scattering additive that is additionally also used within the structure, in order thereby to permit a homogeneous backlighting. In this arrangement the employed scattering additive interferes however in the light-guiding action of the structure, so that ultimately no homogeneous backlighting can be achieved.
In US2007047260 and US2006250819 compound parabolic concentrators on scattering plates for backlight units are described. These however are not linearly arranged, but are circular, tetragonal or polygonal. Also, the CPCs in this case serve only to increase the brightness and not to homogenise the light of a plurality of CCFLS.